We have important news of the operations of the rebel steamer Alabama known as "No. 290." The Cairngorm, an English vessel, lately arrived at Gravesend, from Sydney. She reports that when at Flores, Western Islands, three whale-boats' crews from the Alabama came along-side and reported that their ship, the Ocmulgee, of Edgartown, Massachusetts, had been burned by the Alabama, under command of Captain Semmes, late of the Sumter. The Ocmulgee had two hundred and fifty barrels of oil, and her crew (thirty-four men) were made prisoners. The Alabama had already burned four whalers. She also captured an American schooner (name unknown) in sight of the Cairngorm. The United States sailing sloop of war St. Louis left Lisbon to search, as was supposed, for the rebel privateer "No. 290" off the Azores, in consequence of her raid on American whalers.